1. Field of the Invention
This invention relates generally to a method for the manufacture of molded objects of prefoamed, thermoplastic synthetic material. In particular, by the invention prefoamed, synthetic particles are placed in a mold cavity by means of an air current and are distributed in said mold cavity and further foamed and fused or sintered to each other by the effect of heat and steam, the molded objects thus formed being then cooled by use of the mold surfaces of the mold cavity and removed from the mold cavity after having essentially reached stability of shape. The invention also relates to an apparatus for the execution of the above method, said apparatus being equipped with a split mold containing a mold cavity that is formed between mold wall parts.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn.1.97-1.99
In known methods and apparatus for the manufacture of foamed plastic moldings, such as described in German Printed Application No. 1 504 590, for example, steam is introduced, after the mold wall parts are heated, into the mold cavity between the expanding, prefoamed synthetic particles through nozzles distributed over the mold wall areas, such steam being provided from a steam chamber that is formed on the rear side of the mold wall parts. To cool the mold wall parts and the molded objects formed in the mold cavity, the steam is sucked out of the steam chambers after its introduction into the mold cavity, and cold water is then sprayed against the backside of the mold wall parts. Residual steam in the steam chambers is condensed by the sprayed water. But the steam introduced into the mold cavity, i.e. the steam between the expanding synthetic particles, is also condensed by this cooling operation. The porous moldings thus produced are moist and must be dried more or less before they can be used for their intended purpose.
It was attempted, furthermore, to produce dry, foamed, molded objects or parts by designing the mold cavity so as to be pressure-tight with respect to the steam chamber, whereby the introduction of steam into the mold cavity to fuse the expanding synthetic particles tool place only after the mold wall parts were heated to the point where condensation of the inflowing steam could no longer occur on the parts. The mold walls can be heated either by steam or else by means of a liquid heat-carrying media (see German Printed Application No. 21 29 046 and German Printed Application No. 21 66 710). In these publications the steam required for the fusing or sintering of the expanding synthetic particles is introduced through special steam lines that are separate from the heating and cooling chambers and that utilize special control devices. These known methods and known apparatus necessitate much equipmental cost. Especially, it is difficult and expensive to provide for the transfer of heat to all spots of the mold wall parts with sufficient uniformity.
In another attempt, as described in German Printed Application No. 22 37 397, while providing for the use of steam to heat the mold wall parts and, for that reason, for the formation of steam chambers on the rear side in the mold halves, the steam to be introduced into the mold cavity for fusing or sintering the foaming synthetic particles is not to be transferred into the mold cavity from the heating and cooling chambers. Rather, separate steam chambers are to be formed in the mold halves, whence transfer channels or transfer lines go into the mold cavity via nozzles. However, the mold wall parts in this known method and in this known apparatus are subjected to high pressure loads. Therefore, the mold wall parts must be designed ruggedly with relatively thick walls, again leading to considerable energy losses in view of the heat capacity of the mold wall parts, and causing relatively long cycle times in the machine operation.